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CSCI 558L Lecture 5: Routing In Internet

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  1. CSCI 558LLecture 5: Routing In Internet Internetworking and Distributed Systems Laboratory Young Cho Department of Computer Science University of Southern California Internetworking and Dist. Systems

  2. Routing in the Internet • Routing Information Protocol (RIP) • Open Shortest Path First (OSPF) • Border Gateway Protocol (BGP) • Routing Algorithms and Techniques Routing In Internet

  3. u v destinationhops u 1 v 2 w 2 x 3 y 3 z 2 w x z y C A D B RIP ( Routing Information Protocol) • distance vector algorithm • included in BSD-UNIX Distribution in 1982 • distance metric: # of hops (max = 15 hops) From router A to subnets: Routing In Internet

  4. RIP advertisements • Distance Vectors • Exchanged among neighbors every 30 sec via Response Message (also called advertisement) • Advertisement • Each lists of up to 25 destination subnets within Autonomous Systems (AS) • AS - typically an Internet service provider or a very large organization with independent connections to multiple networks, that adhere to a single and clearly defined routing policy Routing In Internet

  5. RIP: Example Advertisement z Dest Next hops w - 1 x - 1 z C 4 …. … ... w x y A D B C Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x -- 1 …. …. .... A 5 Routing/Forwarding table in D Routing In Internet

  6. RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead • Routes via neighbor invalidated • New advertisements sent to neighbors • Neighbors in turn send out new advertisements • If tables changed • Link failure info quickly propagates to entire net • poison reverse used to prevent ping-pong loops • infinite distance = 16 hops – reports dead link Routing In Internet

  7. routed routed RIP Table processing • RIP routing tables managed by application-level process called route-d(aemon) • Advertisements sent in periodic UDP packets Transprt (UDP) Transprt (UDP) network forwarding (IP) table network (IP) forwarding table link link physical physical Routing In Internet

  8. OSPF (Open Shortest Path First) • “Open”: publicly available • Uses Link State algorithm • Every node constructs a map of the connectivity to the network • Each node independently calculates the next best logical hop • The collection of best next hops form the node's routing table • Unlike distance-vector routing protocols (i.e. RIP) • Only information passed between nodes is connectivity • Route computation using Dijkstra’s algorithm • Shortest path for a graph with nonnegative edge path costs • Produces a shortest path tree • OSPF advertisement carries one entry per neighbor • Advertisements disseminated to entire AS (via flooding) • Carried in OSPF messages directly over IP (rather than TCP or UDP) Routing In Internet

  9. OSPF “advanced” Features • Security • all OSPF messages authenticated • prevents malicious intrusion • Multiple same-cost paths allowed • contrasts to one path in RIP • Different Type of Service (TOS) • satellite link cost set • “low” for best effort • high for real time • Uni- and multicast support • Hierarchical OSPF in large domains Routing In Internet

  10. Hierarchical OSPF • Two-level hierarchy • local area, backbone • LS advertisements only in area • each nodes has detailed area topology; only know shortest path to nets in other areas. • Area border routers • “summarize” distances to nets in own area, advertise to other Area Border routers. • Backbone routers • OSPF routing limited to backbone • Boundary routers • connect to other AS’s. Routing In Internet

  11. Internet inter-AS routing: BGP • BGP (Border Gateway Protocol) • the de facto standard • BGP provides each AS a means to: • Obtain subnet reachability information from neighboring ASs • Propagate reachability information to all AS-internal routers • Determine “good” routes to subnets based on reachability information and policy. • Allows subnet to advertise its existence to rest of Internet: “I am here” Routing In Internet

  12. BGP basics • Difference between BGP and other routing protocols • an external protocol. • BGP is only used by ISP's or large private networks that span across the globe. • Pairs of routers exchange routing information • over semi-permanent TCP connections: BGP sessions • BGP sessions need not correspond to physical links. • BGP4 • An enhanced version of BGP with reduced routing table size RFC 1771 • IBGP • used "inside" the confines of its own AS an autonomous system • works much like a "network neighborhood.“ • EBGP • works the opposite of IBGP • transports information to other BGP enabled systems • generally not used within the same AS Routing In Internet

  13. BGP routes & Path attributes • Advertised prefix • prefix + attributes = “route” • Attributes • Weight • Local Preference • Multi-Exit Discriminator • Origin • AS_Path • Next Hop • Community • Two important attributes: • AS-PATH: contains ASs through which prefix advertisement has passed: e.g, AS 67, AS 17 • NEXT-HOP: indicates specific internal-AS router to next-hop AS. (may be multiple links from current AS to next-hop-AS) • When gateway router receives route advertisement, uses import policy to accept/decline. Routing In Internet

  14. BGP route selection • More than 1 route to some prefix • Router must select route. • Elimination rules • local preference value attribute: policy decision • shortest AS-PATH • closest NEXT-HOP router: hot potato routing • additional criteria Routing In Internet

  15. BGP messages • BGP messages exchanged using TCP. • BGP messages: • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection Routing In Internet

  16. 2c 2b 3c 1b 1d 1c Distributing reachability info • Using eBGP session between 3a and 1c • AS3 sends prefix reachability info to AS1. • 1c can then use iBGP do distribute new prefix info to all routers in AS1 • 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP • Router creates entry for the new prefix in its forwarding table eBGP session iBGP session 3a 3b 2a AS3 AS2 1a AS1 Routing In Internet

  17. legend: provider B network X W A customer network: C Y BGP routing policy • A,B,C are provider networks • X,W,Y are customer (of provider networks) • X is dual-homed: attached to two networks • X does not want to route from B via X to C • .. so X will not advertise to B a route to C Routing In Internet

  18. legend: provider B network X W A customer network: C Y BGP routing policy (2) • A advertises path AW to B • B advertises path BAW to X • Should B advertise path BAW to C? • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers! Routing In Internet

  19. Why different Intra- and Inter-AS routing ? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed Scale: • hierarchical routing saves table size, reduced update traffic Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance Routing In Internet

  20. Routing Algorithms and Schemes • Many Techniques • Key Techniques • Described at the end for you to review • Interesting Details • But not the focus of the course Routing In Internet

  21. Laboratory 3 (HW 1) • Purpose • Learn to use iperf to measure performance • Learn to configure and use DETER • Learn the effects of Bandwidth and Delay • Learn to adjust network specific OS kernel configurations to increase performance • Submission • Create a document with tables, graphs, and paragraphs describing the answers to all the questions • Submit via blackboard Routing In Internet

  22. Next • Next Lecture • IP Router • Adjustments to the Schedules • • Laboratory • Lab 3: Network Performance Measurement Assignment • Lab 1&2 were the DETER/EMULAB tutorials • Use the blackboard ( to submit your document (Due Monday, Feb. 8 at noon) • Reading Assignment • An Integrated Experimental Environment for Distributed Systems and Networks • Use the blackboard ( to submit your slides and Q&A (Due Monday, Feb 8 at noon) Routing In Internet

  23. duplicate creation/transmission duplicate duplicate in-network duplication sourceduplication R4 R3 R4 R2 R2 R3 R1 R1 Broadcast Routing • deliver packets from source to all other nodes • source duplication is inefficient: • source duplication • how does source determine recipient addresses? Routing In Internet

  24. In-network duplication • flooding: when node receives brdcst pckt, sends copy to all neighbors • Problems: cycles & broadcast storm • controlled flooding: node only brdcsts pkt if it hasn’t brdcst same packet before • Node keeps track of pckt ids already brdcsted • Or reverse path forwarding (RPF): only forward pckt if it arrived on shortest path between node and source • spanning tree • No redundant packets received by any node Routing In Internet

  25. (b) Broadcast initiated at D (a) Broadcast initiated at A A A D D G G E B E B F F c c Spanning Tree • First construct a spanning tree • Nodes forward copies only along spanning tree Routing In Internet

  26. A A D D G G B E E B F F c c Spanning Tree: Creation • Center node • Each node sends unicast join message to center node • Message forwarded until it arrives at a node already belonging to spanning tree 3 4 2 5 1 • Stepwise construction of spanning tree (b) Constructed spanning tree Routing In Internet

  27. Source-based trees Multicast Routing: Problem Statement • Goal: find a tree (or trees) connecting routers having local mcast group members • tree: not all paths between routers used • source-based: different tree from each sender to rcvrs • shared-tree: same tree used by all group members Shared tree Routing In Internet

  28. Approaches for building mcast trees Approaches: • source-based tree: one tree per source • shortest path trees • reverse path forwarding • group-shared tree: group uses one tree • minimal spanning (Steiner) • center-based trees …we first look at basic approaches, then specific protocols adopting these approaches Routing In Internet

  29. 1 i 5 4 3 6 2 Shortest Path Tree • mcast forwarding tree: tree of shortest path routes from source to all receivers • Dijkstra’s algorithm S: source LEGEND R1 R4 router with attached group member R2 router with no attached group member R5 link used for forwarding, i indicates order link added by algorithm R3 R7 R6 Routing In Internet

  30. Reverse Path Forwarding if (mcast datagram received on incoming link on shortest path back to center) then flood datagram onto all outgoing links else ignore datagram • rely on router’s knowledge of unicast shortest path from it to sender • each router has simple forwarding behavior: Routing In Internet

  31. Reverse Path Forwarding: example LEGEND S: source R1 router with attached group member R4 R2 router with no attached group member R5 datagram will be forwarded R3 R7 R6 datagram will not be forwarded • result is a source-specific reverse SPT • may be a bad choice with asymmetric links Routing In Internet

  32. Reverse Path Forwarding: pruning • forwarding tree contains subtrees with no mcast group members • no need to forward datagrams down subtree • “prune” msgs sent upstream by router with no downstream group members LEGEND S: source R1 router with attached group member R4 router with no attached group member R2 P P R5 prune message links with multicast forwarding P R3 R7 R6 Routing In Internet

  33. Shared-Tree: Steiner Tree • Steiner Tree: minimum cost tree connecting all routers with attached group members • problem is NP-complete • excellent heuristics exists • not used in practice: • computational complexity • information about entire network needed • monolithic: rerun whenever a router needs to join/leave Routing In Internet

  34. Center-based trees • single delivery tree shared by all • one router identified as “center” of tree • to join: • edge router sends unicast join-msg addressed to center router • join-msg “processed” by intermediate routers and forwarded towards center • join-msg either hits existing tree branch for this center, or arrives at center • path taken by join-msg becomes new branch of tree for this router Routing In Internet

  35. Center-based trees: an example Suppose R6 chosen as center: LEGEND R1 router with attached group member R4 3 R2 router with no attached group member 2 1 R5 path order in which join messages generated R3 1 R7 R6 Routing In Internet

  36. Internet Multicasting Routing: DVMRP • DVMRP: distance vector multicast routing protocol, RFC1075 • flood and prune: reverse path forwarding, source-based tree • RPF tree based on DVMRP’s own routing tables constructed by communicating DVMRP routers • no assumptions about underlying unicast • initial datagram to mcast group flooded everywhere via RPF • routers not wanting group: send upstream prune msgs Routing In Internet

  37. DVMRP: continued… • soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned: • mcast data again flows down unpruned branch • downstream router: reprune or else continue to receive data • routers can quickly regraft to tree • following IGMP join at leaf • odds and ends • commonly implemented in commercial routers • Mbone routing done using DVMRP Routing In Internet

  38. Tunneling Q: How to connect “islands” of multicast routers in a “sea” of unicast routers? logical topology physical topology • mcast datagram encapsulated inside “normal” (non-multicast-addressed) datagram • normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router • receiving mcast router unencapsulates to get mcast datagram Routing In Internet

  39. PIM: Protocol Independent Multicast • not dependent on any specific underlying unicast routing algorithm (works with all) • two different multicast distribution scenarios : • Dense: • group members densely packed, in “close” proximity. • bandwidth more plentiful • Sparse: • # networks with group members small wrt # interconnected networks • group members “widely dispersed” • bandwidth not plentiful Routing In Internet

  40. Consequences of Sparse-Dense Dichotomy: Dense • group membership by routers assumed until routers explicitly prune • data-driven construction on mcast tree (e.g., RPF) • bandwidth and non-group-router processing profligate Sparse: no membership until routers explicitly join receiver- driven construction of mcast tree (e.g., center-based) bandwidth and non-group-router processing conservative Routing In Internet

  41. PIM- Dense Mode • flood-and-prune RPF, similar to DVMRP but • underlying unicast protocol provides RPF info for incoming datagram • less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm • has protocol mechanism for router to detect it is a leaf-node router Routing In Internet

  42. PIM - Sparse Mode • center-based approach • router sends join msg to rendezvous point (RP) • intermediate routers update state and forward join • after joining via RP, router can switch to source-specific tree • increased performance: less concentration, shorter paths R1 R4 join R2 join R5 join R3 R7 R6 all data multicast from rendezvous point rendezvous point Routing In Internet

  43. PIM - Sparse Mode sender(s): • unicast data to RP, which distributes down RP-rooted tree • RP can extend mcast tree upstream to source • RP can send stop msg if no attached receivers • “no one is listening!” R1 R4 join R2 join R5 join R3 R7 R6 all data multicast from rendezvous point rendezvous point Routing In Internet